1. Field of the Invention
The present invention relates to synthesis subband filter processes and apparatuses, in particular, this invention is related to the synthesis subband filtering processes and apparatuses in an audio decoder.
2. Description of the Prior Art
The MPEG (Motion Pictures Experts Group) audio signal specification provides standard encoding/decoding algorithms for audio signals. The algorithms in the MPEG specification can significantly reduce the requirement for data transmitting bandwidths and provide audio signals with low distortions. At present, the encoding/decoding algorithms in the MPEG specification are divided in to three layers: Layer I, Layer II, and Layer III.
The encoding algorithm in the MPEG specification first divides an original audio signal into 32 subband data with an analysis subband filter. Subsequently, based on psychoacoustic models simulating human ears, the encoding algorithm provides signals in different subband with different encoding bit to quantize the signals. After being framed, the quantized signals can then be stored or transmitted.
The decoding algorithm in the MPEG specification is reverse to the steps in the encoding algorithm. The encoded data is first frame unpacked and 32 subband data are then generated with re-quantization. At last, a synthesis subband filter can recover the original audio signal.
Compared with the encoding/decoding algorithms in MPEG-1 Layer I and Layer II specifications, those in the MPEG-1 Layer III (MP3) specification have two more steps. The first one is performing modified discrete cosine transform (MDCT) to the signals outputted from the analysis subband filter. The second one is performing the Huffinan encoding to quantized signals so as to achieve an optimized compression ratio. Correspondingly, the decoding algorithm in the MP3 specification has a step of Huffman decoding and a step of inverse modified discrete cosine transform, (IMDCT).
Synthesis subband filtering is the last step of the decoding algorithm in the MP3 specification. As mentioned in “Coding of moving pictures and associated audio for digital storage media at up to about 1.5 M bits/s” on ISO/IEC 11172-3 Information Technology, the step of synthesis subband filtering in this prior art sequentially converts 18 sets of subband sampling signals after IMDCT into 18 sets of pulse code modulation (PCM) signals; thus, the original audio signal is recovered. Please refer to FIG. 1, which illustrates the flowchart of synthesis subband filtering in this prior art.
Each set of the 18 sets of subband sampling signals after IMDCT respectively includes 32 subband sampling signals. Step S11 is inputting the 32 subband sampling signals being processed. Step S12 is converting the 32 subband sampling signals into 64 converted vectors by matrixing. Step S13 is writing the 64 converted vectors into 1024 default vectors (V) with a first-in, first-out queue. Step S14 is generating a set of first intermediate vectors (U) based on the 1024 default vectors (V). Step S15 is multiplying the set of first intermediate vectors (U) by the 512 window coefficients provided by the MPEG specification to generate 512 second intermediate vectors (W). Step S16 is generating 32 PCM signals based on the 512 second intermediate vectors (W).
As mentioned in “Fast Subband Filtering in MPEG Audio Coding” reported by Konstantinides and Konstantinos, etc. on IEEE Signal Processing Letters 1, 2, Feb. 1994 26-29, 1994, this prior art proposes a method for converting the 32 subband sampling signals into 32 converted vectors by 32-points discrete cosine transform (DCT). That is to say, the matrixing method in step S12 is replaced with 32-points DCT. With the proposed method, the number of converted vectors can be half reduced. The 1024 default vectors (V) are also reduced to 512 default vectors. In this way, the buffer space for storing the default vectors (V) is smaller.
As described above, step S14 through step S16 are generating PCM signals based on the default vectors (V) and the 512 window coefficients provided by the MPEG specification. According to prior arts, before generating the PCM signals, the default vectors (V) must be converted twice, respectively to the first intermediate vectors (U) and the second intermediate vectors (W). However, the conversions not only are complicated, but also require a large number of hardware resources, and takes much time.
Therefore, this invention provides a process and an apparatus for synthesis subband filtering. The process and apparatus according to this invention simplifies the generation of PCM signals into relations between default vectors V and window coefficients D. The problem of complicated calculation in prior arts can thus be solved.